1. Field of the Invention
The invention is generally directed to supplemental oxygen systems, and more particularly, to supplemental oxygen systems for aircraft.
2. Background Description
Modern aircraft operate at altitudes at which there is insufficient oxygen to sustain normal human conscious activities. A recent National Transportation Safety Board Aircraft Accident Brief (NTSB/AAB-00/01 at 6, fn 11) provides background information on this topic:                Pressurized aircraft cabins allow physiologically safe environments to be maintained for flight crew and passengers during flight at physiologically deficient altitudes. (At altitudes above 10,000 feet, the reduction in the partial pressure of oxygen impedes its ability to transfer across lung tissues into the bloodstream to support the effective functioning of major organs, including the brain. These altitudes are typically referred to as “physiologically deficient altitudes.”) At cruising altitudes, pressurized cabins of turbine-powered aircraft typically maintain a consistent environment equivalent to that of approximately 8,000 feet by directing engine bleed air into the cabin while simultaneously regulating the flow of air out of the cabin. The environmental equivalent altitude is referred to as “cabin altitude.”        
Current rules of operation for Transport Category airplanes, FAR 121.333 require a pilot to don and use an oxygen mask whenever the airplane is above 25,000 feet and the pilot is alone on the flight deck, and require at least one pilot to don and use oxygen at all times when the airplane is above 41,000 feet.
Similarly, for pressurized commuter and on demand aircraft operations, FAR 135.89 require a pilot to don and use an oxygen mask whenever the airplane is above 25,000 feet and the pilot is alone on the flight deck, and require at least one pilot to don and use oxygen at all times when the airplane is above 35,000 feet.
These requirements exist because external air pressure at cruise altitude is below the oxygen pressure in the pilot's bloodstream. In the event the cabin lost pressurization, the pilot would rapidly loose consciousness due to hypoxia. The “time of useful consciousness” following a loss of pressurization is shown in Table 1 below.
TABLE 1AmbientPartialPartialTime of usefulpressurepressure ofpressure ofAltitudeconsciousness withoutof21% oxygen50%(ft)supplemental oxygenair (psi)(psi)oxygen (psi)40,00015 seconds 0.571.3635,00020 seconds3.450.73 30,00030 seconds4.360.922.1828,000 1 minute4.771.002.3926,000 2 minutes5.221.102.6124,000 3 minutes5.691.202.8522,000 6 minutes6.201.303.1020,00010 minutes6.751.423.3715,000Indefinite8.29 4.15
Source: “Physiologically Tolerable Decompression Profiles for Supersonic Transport Type Certification,” Office of Aviation Medicine Report AM′ 70-12, S. R. Mohler, M. D., Washington, D.C.; Federal Aviation Administration, July 1970.
An oxygen mask provides a means of supplying 50% or 100% oxygen to the pilot at ambient or near-ambient pressure. Oxygen naturally comprises 21% of the air which, at 15,000 ft., exerts a partial pressure of approximately 1.74 psi. As shown in Table (1) above, the same partial pressure may be provided at 35,000 ft with 50% oxygen, or above 40,000 ft with 100% oxygen (see “Ambient pressure” column above). This is how an oxygen mask provides an extended time of useful consciousness in an unpressurized airplane at cruise altitudes.
During a decompression event at high altitudes, it is conceivable a single pilot, trying to handle an emergency unassisted, could lose consciousness before he or she would be able to don an oxygen mask. Thus the requirement to wear an oxygen mask for any pilot alone on the flight deck.
Even with the development of quick-donning oxygen masks, the brief time between a rapid loss of aircraft cabin pressure and the donning and activation of an oxygen mask may be too long to ensure adequate oxygen for the pilot to safely control the aircraft and avoid losing consciousness. As noted by the NTSB: “Research has shown that a period of as little as 8 seconds without supplemental oxygen following rapid depressurization to about 30,000 feet may cause a drop in oxygen saturation that can significantly impair cognitive functioning and increase the amount of time required to complete complex tasks.” NTSB/AAB-00/01 at 34.
Accordingly, there is a need for improved systems for providing supplemental oxygen to aircraft crew members. The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art.